Overall Reactivation of latent Cytomegalovirus (CMV) remains an important clinical problem following transplantation, despite the prophylactic use of efficacious antiviral agents. Because these target the viral DNA polymerase, they are only active after the virus has reactivated from latency. Their effectiveness is also limited by toxicities and by the emergence of resistant strains. Donor- specific allo-immune tolerance, whereby a transplant recipient is hypo-responsive to donor antigens but continues to have intact immune-responsiveness to other foreign antigens has been the `Holy Grail' of transplantation since the early 1950's. Recently, several experimental protocols have achieved this goal, and the `era of tolerance' seems near. Prior studies have demonstrated that murine CMV (MCMV) reactivation requires the combination of an allo-inflammatory response and immunosuppressive drugs (ISD), while a donor-specific tolerant model would abrogate the inflammatory response, and avoid the need for ISD. Also recently, two central themes have emerged from a combination of studies of animal models of MCMV reactivation and ex vivo/in vitro models of HCMV reactivation: expression of viral genes involved in lytic replication is repressed by epigenetic factors that induce heterochromatinization of viral genomes; and signaling pathways activated by an inflammatory immune response induce reactivation of the virus through epigenetic reprogramming of viral DNA, such that the immediate early genes are released from transcriptional repression, leading to lytic replication. These themes form the basis for studies proposed in 3 separate but inter-related projects studying: 1) Mechanisms of reactivation of latent MCMV following kidney transplantation in a clinically relevant model; 2) Mechanisms of latency and reactivation of HCMV in myeloid lineage cells; and 3) MCMV infection, latency and reactivation in transplantation tolerance. Three Cores will support services needed for the successful completion of studies within these Projects: A) Microvascular surgery and histopathology; B) Precision cell isolation and analysis; and C) Administrative services. The unifying and central hypothesis of the Program Project is that reactivation of latent CMV is induced by inflammatory mediators, which activate signaling pathways, leading to epigenetic reprogramming of viral genomes, resulting in induction of immediate early (IE) gene expression, and ultimately, to reactivation of infectious virus. We believe that blockade of these signaling pathways, either by interfering with specific signaling pathways, blocking epigenetic modifications, or by donor- specific tolerance, prevent reactivation of CMV. We anticipate that integrating mechanistic insights derived from the three projects in this highly collaborative effor will be decisive in moving the field forward and in fostering the development of novel strategies to prevent, rather than treat CMV reactivation.